The present invention relates to an optical recording medium adopting a land and groove recording system and a manufacturing method of a master disk of the optical recording medium which is used to produce a substrate of the medium, and to a cutting device used in the manufacturing method.
Conventionally, a magneto-optical disk of a magneto-optical recording system has been used in practical applications as a re-writable optical recording medium. The magneto-optical disk in general has a groove area (area of a guide groove) and a land area (area between the guide grooves) which are formed into a spiral or a concentric circle on a disk substrate, and either of which includes a recording track. Additionally, a light beam emitted from a semiconductor laser is converged and projected onto the recording track, thereby causing a local rise in temperature of the recording track and recording or erasing information. Further, a light beam whose intensity is such that the information is not erased is converged and projected on the recording track, and by recognizing the polarization state of reflected light, the information is reproduced.
Further, among methods of obtaining address information in the magneto-optical disk, a method employing a pit series is commonly adopted. In this method, the pit series is formed continuously on the recording track of the magneto-optical disk, and address information is obtained by reproducing change in quantity of the reflected light from the pit series.
Meanwhile, lively studies and developments with respect to magnetic super resolution reproduction using a multilayer magnetic film have largely been improving a reproducing resolution of the magneto-optical disk of a super resolution magneto-optical recording system, and active research on a system of recording information on both of the groove and land areas (land and groove recording system) has been carried out.
Incidentally, in the land and groove recording system, since the recording track is formed for each of the land and groove areas, address pits which correspond to the respective areas are required. FIG. 14 is an explanatory drawing showing a structure of a magneto-optical disk substrate disclosed in Japanese Unexamined Patent Publication No. 161077/1995 (Tokukaihei 7-161077 published on Jun. 23, 1995). As shown in FIG. 14, in this magneto-optical disk substrate, address pits P1 and P2 are formed for a spirally formed groove area G and land area L, respectively, which have the same depth from surfaces of the respective areas G and L, thereby obtaining the respective address information of the groove area G and land area L.
Further, FIG. 15 is an explanatory drawing showing a structure of a magneto-optical disk substrate disclosed in Japanese Unexamined Patent Publication No. 28729/1994 (Tokukaihei 6-28729 published on Feb. 4, 1994). In this magneto-optical disk substrate, a series of address pits P1 having the same depth as that of the groove area G is formed on the groove area G having a spiral shape. On the other hand, on a portion of the land area L away from the portion where the series of address pits P1 are provided, a series of address pits P2 having the same depth as that of the groove area G are formed.
By thus forming the series of address pits P1 and P2 at separate positions, crosstalk caused by an adjacent address pit can be reduced, thereby obtaining accurate address information.
However, a technique disclosed in the above Publication (Tokukaihei 7-161077) requires two or three types of photoresist having different photosensitivities, and an intermediate layer in a complicated exposing and developing process in order to create the groove and address pits in the magneto-optical disk substrate.
Further, the magneto-optical disk substrate disclosed in the publication (Tokukaihei 6-28729) raises such a problem that, since the address pit and groove have the same depth, a reproduced signal from the address pit becomes small, and address information cannot be reproduced accurately.
It is an object of the present invention to provide an optical recording medium which is easily manufactured and can accurately reproduce address information, and a manufacturing method of an optical recording medium master disk which is used to produce a substrate (optical recording medium substrate) to be used for the foregoing medium, and a cutting device used in the manufacturing method.
In order to attain the foregoing object, an optical recording medium of the present invention present recording medium) includes a groove having a first address pit and a land having a second address pit, in which a groove depth dG, a first address pit depth dP1 and a second address pit depth dP2 are set to satisfy
dG less than dP1≈dP2xe2x80x83xe2x80x83(a).
The present recording medium is an optical recording medium, including an optical disk and a magneto-optical disk such as CD (Compact Disc), for recording and reproducing information by the exposure of a light beam. Moreover, as explained, the present recording medium includes the address pits in both of the groove and land, thereby being an optical recording medium of a land and groove recording system in which both groove and land have the recording tracks.
Particularly, in the present recording medium, as indicated by equation (a) above, the first address pit provided in the groove and the second address pit provided in the land are made deeper than the groove depth. This enables a reproduced signal from these address pits to be intense, thereby reproducing address information accurately and stably.
Further, in the present recording medium, as indicated by equation (a) above, the depth (from the surface of the land) of the first and second address pits are set at values substantially equal to each other. Consequently, these two kinds of address pit can be manufactured by a single manufacturing process, and thus the present recording medium can be manufactured by a relatively facile process.
In order to attain the foregoing object, a manufacturing method (present manufacturing method) of an optical recording medium master disk in which address pits are provided in both of the land and groove, includes the steps of: (a) applying a resist over a master disk substrate; (b) performing exposure and development with respect to the substrate so as to partially maintain the resist in a groove forming area, while avoiding remaining the resist in a pit forming area; (c) etching a surface of the substrate which is uncovered in the pit forming area; (d) uncovering the surface of the substrate in the groove forming area; and (e) etching the surface of the substrate in both of the groove and pit forming areas.
The present manufacturing method is for manufacture of an optical recording medium master disk (hereafter simply referred to as master disk) which is used to manufacture a substrate used in the optical recording medium. Namely, the foregoing substrate of the optical recording medium is manufactured by injection molding of a substrate material such as resin, in which a stamper obtained from the master disk is used as a mold.
Consequently, the master disk has the same arrangement as that of the substrate of the optical recording medium (including a groove, land and address pit, etc.).
Further, as explained, the present manufacturing method is set to manufacture the master disk having the address pits in both of the groove and land. Therefore, the substrate of the optical recording medium which is manufactured from this master disk is to be adopted in an optical recording medium of a land and groove recording system.
Further, the foregoing groove forming area refers to a portion of a master disk substrate (substrate to be the master disk) where the groove is formed, and likewise, the pit forming area refers to a portion of the master disk substrate where the address pit is formed.
Meanwhile, in the present manufacturing method, the step (b) is set such that the resist partially remains in the groove forming area, while preventing the resist from remaining in the pit forming area.
Further, in the first dry etching process performed after this step, the surface of the substrate which appeared in the pit forming area is etched. On the other hand, since the resist is remaining in the groove forming area, etching by the first dry etching process is avoided therein.
Therefore, at the end of the first dry etching process, the pit forming area of the master disk substrate is deeper by the depth of etching than the groove forming area.
Then, in the present manufacturing method, after the residual resist in the groove forming area is removed by the first ashing process, the groove and address pit are formed by the second dry etching process by etching the master disk substrate appeared in the groove forming and pit forming areas.
As explained, in the present manufacturing method, only the pit forming area is etched in the first dry etching process, while both of the groove forming and pit forming areas are etched in the second dry etching process. Namely, in the present manufacturing method, the groove forming area is etched only once, while the pit forming area is etched twice, thereby forming the address pit deeper than the groove.
Further, in the present manufacturing method, the groove and address pits are formed by etching the master disk substrate appeared in the groove forming and pit forming areas. Accordingly, the groove and address pits are formed into a flat rectangular shape, and it is possible to manufacture the optical recording medium master disk which can suppress groove noise of the optical recording medium, compared with a case where the groove and address pits were formed on the resist surface having fine irregularities.
Further, in the present manufacturing method, the resist applied onto the master disk substrate is a single layer without an intermediate layer which is to be removed afterward, thereby efficiently manufacturing the master disk at low cost.
Additionally, since the depth of the groove and address pits of the master disk can be determined only by the etching conditions of the first and second dry etching processes, the master disk having a desired shape can be manufactured with good reproducibility.
Note that, it is preferable that the present manufacturing method includes a second ashing process to remove the residual resist on the master disk substrate after the second dry etching process. The xe2x80x98ashingxe2x80x99 here refers to a process such as dry etching employing oxygen gas plasma for example, which prevents the surface of the master disk substrate from being further etched, while removing by ashing the resist on the master disk substrate.
Further, in the present manufacturing method, it is preferable to utilize the following cutting device: the cutting device having an arrangement which includes a beam splitter for splitting the light beam which is emitted from the light source into first and second light beams; an objective lens for converging the first and second light beams; and a numerical aperture reducing section for reducing the numerical aperture of the objective lens with respect to the first light beam.
With this cutting device, the first and second light beams can be generated from a single light source, and the numerical aperture of the objective lens with respect to the first light beam can be reduced by the numerical aperture reducing section.
The spot diameter of the light beam becomes xcex/NA when xcex is the wavelength of the light beam, and NA is the numerical aperture with respect to the light beam. Therefore, by reducing the numerical aperture, the spot diameter of the light beam can be increased. Accordingly, by using the foregoing cutting device, the first and second light beams having different spot diameters can readily be generated.
Note that, as the numerical aperture reducing section, for example, a light shielding plate which reduces the diameter of the first light beam can be adopted.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.